Electrode-shunting method of producing electrophotographic pictures and apparatus therefor

ABSTRACT

An electrophotographic method and apparatus therefor. The method is based on optically forming a conductivity image of a master on or in a photoconductive layer which is part of one of two spaced electrodes. An electric field is applied across the space between the electrodes during the formation of the conductivity image causing a migration of charged toner particles in a dispersion occupying the space between the electrodes. When the field is removed, and the electrodes are shunted a further migration occurs to form a toned image of the master on an image support at one side of the space. Positive or negative images can be formed according to the direction of the field and the sign of the charged particles.

United States Patent Mohn et al. 1 Aug. 26, 1975 ELECTRODE-SHUNTING METHOD OF 3,681,064 3 1972 Yeh 96/] PE PRODUCING ELECTROPHOTOGRAPHIC W ig F uagln l2l.... PICTURES AND APPARATUS THEREFOR 3,776,722 12/1973 Cantarano 96/1 R [75] Inventors: Eugen Mohn, Egg; Hansjurg Hermann, Herisau, both of Switzerland [73] Assignee: Turlabor AG, Zumikon, Switzerland [22] Filed: June 26, 1973 [21] Appl. No; 373,823

[30] Foreign Application Priority Data June 30, 1972 Switzerland H 009829/72 [52] U.S. Cl. 1. 96/1 PE; 96/1 R; 204/181 PE; 355/3 P; 355/4; 355/17 [51] Int. Cl. G03g [58] Field of Search 355/3 P, 4, l0, l7; 96/].2, 1,3, 1 PE, 1 R; 117/37 LE; 204/181 PE [56] References Cited UNITED STATES PATENTS 3,346,475 10/1967 Matkan et al 96/13 X 3,438,706 4/1969 Tanaka et a]. 355/1 l Primary ExaminerRichard L. Moses Altorney, Agent, or Firm-Armand G, Guibert; Milton M. Wolson 57 ABSTRACT An electrophotographic method and apparatus there for The method is based on optically forming a conductivity image of a master on or in a photoconductive layer which is part of one of two spaced electrodes. An electric field is applied across the space between the electrodes during the formation of the conductivity image causing a migration of charged toner particles in a dispersion occupying the space between the electrodes. When the field is removed, and the electrodes are shunted a further migration occurs to form a toned image of the master on an image support at one side of the space. Positive or negative images can be formed according to the direction of the field and the sign of the charged particles.

45 Claims, 8 Drawing Figures PATENTEDAUGZS IQYE, 3,901,696

sum 2 u? 5 ELECTRODE-SHUNTING METHOD OF PRODUCING ELECTROPHOTOGRAPHIC PICTURES AND APPARATUS THEREFOR The present invention relates to a method of producing pictures. The invention refers to an apparatus for carrying out the method and to a use of the method.

For the production of pictures various electrophotographic methods are already known. In them a photoconductive layer is imparted a uniform electrostatic charge and then exposed in accordance with a master copy. By the selective discharge which takes place in this connection as a function of the master copy there is produced a latent electrostatic charge image which corresponds to the master copy. This latent image can be developed directly by known methods and then possibly transferred to a support by the so-called image transfer process However, it is also possible to transfer the charge image to a suitable support by the so-called charge transfer process before it is developed and only then develop it.

However, there have also been proposed electrophotographic processes which operate without the production of a latent electrostatic charge image. These methods are based on the variable conductivity of a conduc tivity image corresponding to a master copy and produced in or on a photoconductive layer and electrolytic production of the image (cf. US. Pat. No. 3,130,665). It has also been proposed to produce an electrostatic charge image on the surface ofa photoconductive layer as a function of such conductivity image by means of a semiconductive layer which is in close contact with the photoconductive layer (cf. UK patent No. 797,027).

Finally, it is known to produce an image on a semiconductor by electrophoretic precipitation of colloidal particles (see the article methods for Producing Photographic Images on Semiconductor Surfaces by Precipitation of Colloidal Particles and by Electrophoresis, by N. l. Bochkareva, L. G. Paritskii and S. M. Ryvkin, A. F. Ioffe, Physicotechnical Institute, Academy of Sciences of the USSR, Leningrad, translated from Fizika i Tektnika Poluprovodnikov, Vol. 5, No. 5, pp. 900 903, May I971).

Known electrophotographic processes for producing images have considerable disadvantages, particularly when they serve for the production of colored images, for instance:

I. A high-voltage corona is required for the charging of the photoconductive layer.

2. The high-voltage corona results in damage or impairment to the photoconductive layer.

3. Undesired production of ozone.

4. Several transfer steps are necessary.

5. The production of exact images is difficult or complicated.

6. The exact reproduction of half tones and mixed colors is very difficult due to the steep photographic characteristic in these processes.

7. If a paper provided with a photoconductive layer is used in order to circumvent transfer steps, extremely complicated toner material is necessary in order to master the difficulties of the electrophotographic process as a result of the first and second partial images which have already been produced on the paper. Electrolytic development of a conductivity image gives an image on the surface of the photoconductor, which image must then be transferred in an image transfer process. Furthermore, in this case each step. i.e. each partial image, requires a large amount of time, for instance about 20 minutes.

8. C harge-transfer methods, while they permit almost simultaneous exposure and development, nevertheless require image transfers onto the final support which affords considerable difficulties in the production of exact images, for instance with respect to the exact covering of the individual partial color images.

The object of the present invention is to provide a simple and rapidly operating method for the rapid and precise production of images. preferably for colored images, and also an easily handled apparatus for carrying out the process which does not have the said disadvantages.

The present invention relates to a method of producing images in accordance with a master, which method is characterized by the fact that by exposure, as a function of the master, of a photoconductive layer onto or into said layer, there is produced a conductivity image which corresponds to the master and which is utilized to control, within a space adjacent said layer, an electrical field through which charged toner particles are moved, whereby the charge of toner particles which contact the said layer or a support for an image close to it is varied, and in a further method step, upon the disconnecting of the said electrical field, toner particles migrate in accordance with the master to the support for the image.

The invention also relates to an apparatus for the carrying out of said process which is characterized by a first electrode and a second electrode, which electrodes may be optionally connected to a source of voltage or to each other, the first electrode having associated with it a photoconductive layer which can be exposed by imaging means in accordance with a master, and a support for an image is arranged on the second electrode and a medium having charged toner particles can be introduced in the space between the two electrodes.

The invention also concerns the use of the said process for producing single-colored or multi-colored pictures.

The invention furthermore relates to pictures produced by the said method.

The invention will be explained in further detail below with reference to the drawing in which:

FIG. 1 is a schematic showing of the apparatus, not drawn to scale,

FIGS. Ia, lb, 10 show three phases of the generation of the image (cross-hatching to represent transparent material, such as glass, being omitted in element 2 for clarity of FIG. lb,

FIG. 2 shows a second diagrammatic embodiment,

FIG. 3 shows details of a first holding device of embodiment of FIG. 2,

FIG. 4 shows a portion of FIG. 3,

FIG. 5 shows details ofa second holding device of the embodiment of FIG. 2.

In all figures, corresponding parts have been provided with the same reference numbers.

FIG. I is a diagrammatic view of a first embodiment of an apparatus for the carrying out of the said method. This figure shows merely the basic construction of the arrangement in order to explain the manner of operation and is not drawn to scale.

A first electrode 1 is formed by a transparent glass plate 2 which is covered on one side by a thin, transparent. conductive layer 3. This conductive layer 3 can consist, for instance. of tin oxide, as is the case in the known NESA glasses, On the conductive layer 3 there is applied a photoconductive layer 4, for instnace of panchromatically sensitized poly-(N)-vinyl carbazole. The sensitizing can be effected, for instance, by adding a small amount of tetranitrofluorenone (TNF). The thickness of the photoconductive layer 4 is so selected that it absorbs light in the visible spectral range and does so in such a manner that the light activates the layer in its entire thickness. For a layer of polyvinyl earbazole, the thickness of the layer is preferably about 10 to 100 pt.

Facing the photoconductive layer 4 of the electrode 1 and equidistant at a slight distance therefrom. for instance a few tenths of a millimeter, there is arranged a second electrode 5, for instance an aluminum plate. A support 6 for an image. for instance a sheet of paper, lies on the second electrode 5 in good contact with it. The distance d between the photoconductive layer 4 and the image support 6 is preferably not greater than about 300 t and at most about 2mm.

As support 6 there is suitable a large number of materials which have suitable values of electric volume resistance and surface resistance, Thus. for instance, nor mal typewriter paper is well suited. but a plastic sheet, a plate of glass. a metallic foil or a textile material such as a woven or knitted fabric can also be used.

In the space 7 between the photoconductive layer 4 and image support 6 there is contained as developing medium a dispersion 8 of colored toner particles 9 in an organic liquid 10 of high ohmic value which assumes a given ZETA potential as a result of the addition of a charge-control agent. lnformation as to suitable materials will be given later on.

From a master 11, for instance a slide, an image is projected by optical focusing means 12 through the transparent electrode 1 onto the photoconductive layer 4. As imaging means 12 there is suitable, for instance, a slide projector having a source of light l2A. a condenser l2B, a color slide as master ll and an objective 12C, which are only diagrammatically indicated in FIG. 1. However, other imaging means of known type can be used which produce an image corresponding to a master 11 on the photoconductive layer.

For the reproduction of colored pictures, suitable partial color pictures and toner materials are used in succession. Further information will be given later on.

The electrode l of the photoconductive layer 4 is connected via a switch 13 either to the one terminal, in the present example the negative terminal. of a source of voltage 14. or to the second electrode 5. The second electrode 5 is connected to the other terminal, in this case to the positive terminal of the source of voltage l4, and preferably also to ground.

The dispersion 8 contains toner particles 9 preferably of the order of magnitude of (H to 50 t. The organic liquid 10 preferably has a resistivity of about lO' IU ohm cm. As color for the toner particles 9 there is se' lected for the production of colored images preferably in each case one of the subtractive primary colors cyan. magenta or yellow. The toner concentration can may approximately within the range from 0.] to 10 percent by weight.

The distance between the support 6 and the photoconductive layer 4 is preferably approximately in the range of 20 p, to 500 a. At least during a part of the exposure time of the photoconductive layer 4 an electric field having a fiedl strength of about 1 to l00 volts/AL is present between the two electrodes 1 and 5, depending upon the sensitivity of the photoconductive layer 4 as well as the charge of the toner particles. Color filters for the image produced on the photoconductive layer 4 are to be provided for the production of colored images, namely for the additive primary colors red, green and violet. There are suitable for this purpose, for instance, the following Agfa-Gevaert separation filters:

red [.599 green U525 violet U438 During the exposure time which, depending on the intensity, lies within the order of magnitude of about a few tenths of a second to a few seconds. a voltage amounting to about 500-3000 volts is applied by means of the switch 13 to the electrodes 1 and 5 in order to produce the said electrical field. After the exposure the electrical field is disconnected and the two electrodes are connected directly with each other by actuating the switch 13. As a result of these switch operations. due to the action of the electrical field, first an extremely rapid migration of the charged toner particles takes place, Upon contact with the photoconductive layer 4 or with the support 6, changes in charge of the corresponding toner particles occur. Then, after removal of the previously applied voltage, i.e. after the two electrodes are connected with each other, a further migration of toner particles takes place relative to the surface of the support 6 on which there is deposited a partial color image which corresponds to the conductivity image, corresponding to the master ll, of the photoconductive layer 4.

A possible explanation for the conditions and processes occurring in the space 7 will be given now with reference to FIGS. la, lb. and It.

In the first phase. shown in FIG. la, the two electrodes l and 5 are directly connected with each other. i,e., shunted, via the switch 13 which is in its position l3, The colored toner particles 9 are in any desired distribution in the liquid 10. In the present example these toner particles are assumed to have a positive charge.

In the second phase, shown in FIG. lb, the two electrodes l and 5 are connected to the two poles of the source of voltage 14 via the switch 13 which is now in its position 13''. As indicated by the vertical arrows in this figure, a pattern of light from the imaging means (not shown) will fall on electrode I. As a result of the exposure of the photoconductive layer 4, there is produced a conductivity image which corresponds to the specific color separation of the master 1]. Below the structure ofthe electric field in the space 7 which is de pendent on said conductivity image. the positive toner articles 9 (cf. FlG. lu) migrate in the direction towards the layer 4 and only those which strike a place on the photoconductive layer which has become conductive by the exposure will give off their positive charge. The particles 9' which impinge upon the noneonductive places retain their charge.

In the third phase, shown in FIG. 10, the two electrodes l and 5 are again shunted, i.e., directly con nected with each other by the switch 13 which has been brought again into position I3 and the positive particles 9' migrate in the direction towards the electrode 5 which is at zero potential, thus forming on the support 6 a color deposit 9" corresponding to the conductivity image of the photoconductive layer 4.

The formation of this color deposit takes place very rapidly, Le. in less than 1 second ater bringing the switch 13 back into its original position [3. The behavior of the toner particles has been described above with reference to FIGS. la to In only with regard to the two particles 9 or 9 or 9". Of course. similar behavior applies to the other particles in the dispersion 8.

Thereupon the image support 6 is preferably pulled off laterally, for instance together with the electrode 5 along appropriate guide members (not shown, but described in greater detail subsequently). The first partial color image obtained is dried preferably on its surface, and returned along the guid members to the original position whereupon the process is repeated with another color filter and corresponding toner material and the third partial color image is produced on the image support 6 in analogous fashion with a third color filter and third toner material.

For the production of the three partial color images, there is preferably maintained a specific sequence depending on the properties of the toner materials, for instance the sequence yellow, magenta, cyan, which is customary in conventional color printing technique.

The photoconductive layer is preferably protected by a thin protective layer against mechanical and chemical attack in order to obtain a longer life of use for this layer. Teflon, SiO or else silicon nitrides are suitable, for instance, for this protective layer.

Of course, the photoconductive layer 4 on the first electrode 1 must be cleaned before each change in color, which, however, can easily be done due to the merely small quantity of dispersion 8.

The finished color image produced with all three partial color images can furthermore be sprayed with a colorless lacquer in order to fix the image and increase its transparency and then it can be dried, for instance by passing it under a source of heat radiation.

For the protection of the photoelectric layer 4 during its exposure an infrared filter can preferably be provided, whereby the photoconductive layer 4 is protected against warming up. At the same time an im provement of the reproduction of red in colored pictures can be obtained hereby.

The method described has a number of advantages over known image-producing methods which do not occur in this combination in any of the known methods, namely:

I. Possibility of being able to use ordinary paper as image support.

2. No charging of a photoconductive layer necessary,

3. No development of ozone,

4. No problems in obtaining an exact registering of partial color images since the relative position of the original and of the copy is unchanged during each color process and can be obtained for the different partial color images by simple mechanical stops, The picture support 6 remains on the electrode 5 unchanged in position relative thereto for all partial color images,

5. Excellent reproduction of half tones and mixed colors,

6. Direct production of images on the final image support without transfer steps,

7. Separation of image on the support during the breakdown itself very short of the electrical field within a fraction of a second.

Since on basis of FIG. 1 the development and manner of operation of the first embodiment of an apparatus for the carrying out of the said method has been described and explained, a second embodiment will now be described with reference to FIGS. 2 to 5.

In accordance with the second embodiment, various method steps necessary in the course of the production of a colored picture are made automatic.

The apparatus shown in FIG. 2, is provided, as imaging means 12, with a slide projector the construction of which is known and which is therefore only shown schematically. A color slide serving as master 1] is inserted into the slide projector in the direction indicated by the arrow IS. A shutter I6 is arranged behind the imaging means 12. As shutter I6 there can he used a shutter which is known from use in cameras. The shutter 16 serves to set the exposure time necessary for producing the conductivity image.

One of three color filters I8, 1), 20 can be interposed in the beam path 17 of the slide projector, Each of these color filters serves to produce the corresponding color separation or partial color image. As color filters 18, I) and 20 there are suitable, for instance, Agfa- Gevaert separation filters red: L599, violet: U43 8, and green: U525. Preferably there can also be provided a gray filter 2] which can be inserted where necessary in the path of the beam of light in order to adjust the intensity of illumination in the case of masters of differ ent average brightness in such a manner that the same exposure time is always required, regardless of the master.

Via a mirror 22 which is inclined at 45 to the horizontal, the beam path I7 is deflected downward onto the first electrode I bearing the photoconductive layer 4, this electrode being arranged horizontally in a first holding device 23. By means of a line 24 the first electrode 1 and its conductive layer 3 are connected with the switch 13. The first electode I can be connected via the switch 13 either with the one terminal 14A, in this case the negative terminal, of the source of voltage 14 or via a line 25 with the second electrode 5. The second electrode 5 is also connected with the second terminal 148, in this case the positive terminal of the source of voltage 14., and with ground. The second electrode 5 is conductively connected with a second holding device 26. The second holding device 26 is supported for displacement in the direction indicated by the doubleendcd arrow 28 horizontally by guide means such as guide rails 27. The guide rails 27 are supported by hear ing pedestals 29 on a base plate 30 of the apparatus 100. On the lefthand side of the guide rails 27 there is provided a stop 31 which is preferably adjustable and by means of which the left end position of the second holding device 26 can be adjusted in precisely reproducible manner.

On the righthand side of the guide rails 27, a loading device 32 is provided. By means of the loading device 32, a sheet of paper can be fed from a supply roll 33 to the second electrode for each image to be produced. By means of driven transport rollers 34, a strip of paper wound on the supply roll 33 is pulled off. By a cutting device 35 which is automatically actuated as a function of the desired format of paper, a corresponding piece is cut off in each case from the said strip and pushed by means of further driven transport rollers 36 onto the electrode 5 of the second holding device 26 in its righthand end (dotted line) position. The second holding device 26 is preferably connected with a vacuum device (not shown in FIG. 2) by means of which the new sheet of paper is fixed on the electrode 5 over suction holes. The apparatus 100 preferably also has a wetting means 37. The wetting means 37 include a storage tank 38 for the wetting agent 39, for instance Isopar G manufactured by the Esso Company.

By the wetting device 37 the paper 6 is saturated with said wetting agent 39 so that upon the introduction of the toner no solvent is removed from it by the paper since its concentration would be changed thereby.

The storage tank 38 is connected via a line 40 with a pump 4i which feeds the wetting agent 39 via a second line 42 to a two-way valve 43. As two-way valve 43 there is suitable an electromagnetically switchable valve of known construction. From the two-way valve 43, a line 44 extends to a nozzle arrangement 45 under which the second holding device 26 can be passed.

The apparatus 100 furthermore has a multiple toner delivery device 46. Each individual color has its own dispensing part associated with it. A first storage tank 47' contains a supply of the toner 48' for the first individual color. Via a line 49' the toner 48' flows to a toner pump 50' and is fed by the latter via a line 51' to a two-way valve 52'. As two-way valve 52' there is preferably provided an electromagnetically actuatable valve. Upon the actuation of the two-way valve 52' it conducts the toner 48' via a line 53' to an injection nozzle 55' at the end of the line 53. Upon nonactuation of the two-way valve 52 it conducts the toner 48' via a line 55' back into the storage tank 47'.

In analogous fashion there are provided separate storage tanks 47" and 47", toner pumps 50" and 50", two-way valves 52" and 52", and injection nozzles 54" and 54". and the corresponding lines 53", 53" for the toner 48" and 48". for the second and third individual colors; cf. FIG. 4.

When the second holding device 26 is in its lefthand end (full line) position, there is a gap of, for instance, about 300 p. between the photoconductive layer 4 of the first electrode l in the first holding device 23 and the second electrode 5 in the second holding device 26.

The apparatus 100 also has a cleaning device 56 ar ranged lateral to the first holding device 23. as well as a drying device 57, the second holding device 26 being moved past both of said devices. The cleaning device 56 contains a roller 58 which is movable downward. for instance electromagnetically. said roller being preferably provided with an elastic covering such as rubber or the like. The drying device 57 can be provided. for instance, with heat radiators and possibly with a blower,

FIG, 3 shows details of the first holding device 23. The first electrode 1, with the photoconductive layer facing downward, is mounted in a frame 59. In the side part of the frame 59 there are arranged the ends of the lines 53', 53" and 53" to which the injection nozzles 54'. 54" and 54" (not numbered in FIG. 3) respectively are connected.

As can be noted from FIG. 4 which shows a portion of FIG. 3 on a larger scale, the injection nozzles 54', 54" and 54" are so directed with respect to an oblique edge 60 of the first electode 1 that the jet of toner liq uid emerging from them can flow into the space 7 (not shown in FIG. 4) lying below the photoconductive layer 4 and above the second electrode 5.

FIG. 5 is a detail view of a second holding device 26. 61 represents a wiper which consists, for instance, of a profiled rubber blade provided with a sharp edge. The wiper 61 serves to clean the photoconductive layer 4 after the production of each partial color image during the movement of the second holding device 26 towards the right. A cleaning roller 62 also serves for the cleaning of the photoconductive layer 4.

Since the construction of the apparatus has now been described with reference to FIGS. 2 to 5, a de scription will be given of the various method steps for the production of an image composed of three individual colors.

I. The second holding device 26 is brought into its righthand end position. The pump 41 and the pumps 50', 50" and 50" are placed in operation and the lamp 12A of the slide projector is energized.

2. By briefly connecting the drive member for the transport rollers 34, a length of the strip of paper wound on the supply roll 33 is pulled off.

3. After reaching a given length, a suitable sheet of paper is cut from the paper strip by a brief actuating of the cutting device 35.

4. The cut sheet is pushed onto the electrode 5 on the holding device 26 by the connecting for a brief time of the drive member for the transport rollers 5. The vacuum device of the second holding device 26 is connected, the sheet of paper is drawn onto the electrode 5 and thereby fixed in its position on the electrode 5.

6. The second holding device 26 is moved to the left, for instance by a motor acting via a rope, a rack or a threaded spindle.

7. Only during the passage of device 26 past wetting device 37 is the two-way valve 43 actuated in such a manner that wetting agent 39 is sprayed from the nozzle arrangement 45 onto the surface of the sheet of paper lying on the electrode 5.

8. Upon the first movement of the second holding device 26 below the first holding device 23, the twoway valve 52' is actuated to the full-line position shown in FIG. 2 for the feeding of toner 48' for the first color so that during the further movement of the second holding device 26 until it reaches the stop 31, the space produced between the photoconductive layer 4 and the second electrode 5 actually the sheet of paper 6 present on it is filled with the first toner 48'.

9. The color filter 18 for the first color is brought into the light path 17.

It). The switch 13 is switched to the terminal 14A of source of voltage 14 in order to produce the electrical field in the space between the two electrodes.

11. The shutter 16 is opened during the exposure time T which is optimum for the first color, whereby a conductivity image corresponding to the first individual color image of the master is produced in and on the photoconductive layer 4.

12. The switch 13 is switched in such a manner that now the first electrode 1 is connected with the second electrode 5. in a fraction of a second there takes place a deposit of color corresponding to the first individual color image on the support 6, i.e. on the sheet of paper lying on the electrode 5.

13. The second holding device 26 is pulled away towards the right laterally from the first holding device 23, whereby the wiper 61 and the cleaning roller 62 remove the toner 48' from the photoconductive layer.

14. The cleaning devicd 56 is temporarily lowered onto the passing support 6 and thus onto the sheet of paper bearing the first color deposit, whereby excess toner 48 which is still adhering to the support 6 is wiped off.

15. The second holding device 26 continues to travel towards the right to a point below the drying device 57 which is temporarily activated and the first partial color image is thereby dried on its surface.

16. During the drying process the cleaning device 58 is at the same time freed of any adhering toner by the cleaning roller 62.

17. Upon completing the superficial drying. the first cycle during which the first partial color image has been produced is concluded.

In the secondy cycle the second color image is produced in a manner similar to production of the first color image.

18. The holding device 26 is moved towards the left, and during its travel below the first holding device 23, the toner 48" for the second partial color image is fed by actuation of the branch valve 52''.

19. The color filter 19 for the second partial color image is brought into the beam path 17.

20. The switch 13 is switched to the terminal 14A of the source of voltage 14.

21. The shutter 16 is opened for the exposure time T which is optimum for the second color, a conductivity image corresponding to the second color image being produced in and/or on the photoconductive layer 4.

22. The switch 13 is switched in such a manner that the first electrode 1 is connected with the second electrode 5. In a fraction of a second there is produced a deposit of color corresponding to the second partial color image on the support 6, i.e. on the sheet of paper already provided with the first color image, the sheet 6 resting on the electrode 5.

23. The second holding device 26 is pulled away towards the right laterally from the first holding device 23, the wiper 61 and the cleaning roller 62 removing the toner 48" from the photoconductive layer 4.

24. The cleaning device 56 is lowered onto the passing support 6 (the sheet of paper bearing the first and the second color deposits), excess toner 48" still adhering to the support 6 is wiped off.

25. The second holding device 26 continues to travel to the right to a point below the drying device 57 which is temporarily activated and the second partial color image is thereby dried on its surface.

26. During the drying process, the cleaning device 58 is simultaneously freed from any adhering toner by the cleaning roller 62.

27. Upon the drying of the surface, the second cycle during which the second color image has been applied to the color image is now also concluded.

ln similar fashion. one now proceeds with the third 5 toner 48" and the third color filter and the optimum exposure time T for the third partial color image of the third cycle for the production of the third partial color image on the support 6 already bearing the first and the second partial color images, steps 28 to 37 now following analogously corresponding to steps 18 to 27.

38. Finally the superficially dried support 6 bearing all three color images can be coated with an agent which increases the transparency and durability of the complete color image.

39. This is followed by a final drying, whereupon the colored picture to be produced is finished.

The above steps 1 to 38 can be achieved by manual actuation of the various parts of the device 100 indicated or else by automatic actuation thereof. There is no difficulty in designing the corresponding drives and controls on basis of the steps which have been described in detail. Thus the individual switch times for the movement of the second holding device 26, for the placing in operation of the coating device 32, the wetting device 37, the toner feed device 46, the cleaning device 56. the drying device 57 and the replacement of the color filters can be controlled. for instance, by means of a cam-controlled programming device.

For the production of the photoconductive layer 4 the following materials enter, for instance, into consideration:

Organic polymers, such as polyvinyl carbazole; organic pigments in a suitable layer support, such as phthalocyanin. or inorganic pigments in suitable layer supports, for instance cadmium sulfide, zinc oxide. etc. The layer support consists of a suitable binder, for instance of acrylic resin, styrene resin or similar substances.

In the toner 48', 48" and 48" the particles may be charged positively or negatively. In case of reversed polarity of the NESA glass acting as electrode 1 there is produced corresponding to the particles on the support 6 a positive image and in case of the same polarity a negative image if a positive is used in this connection as master. ln corresponding manner, a negative master can be converted into a positive image. As support 6 there can be used, for instance. ordinary typewriter paper; plastic foil, for instance Mylar; textile material, whether woven or knitted; glass plate, aluminum foil or some other metal foil or plate.

In order to obtain good image transparency, a filmforming material is preferably applied to the support 6 after the production of the three individual color images. As such film-forming material there is suitable a lacquer. for instance any acrylic lacquer; a resin, for instance styrene resin; a low-melting polymer, for instance polyethylene'. or a wax. for instance paraffin wax. The application can be effected by customary technique, for instance by spraying. coating, dipping or rolling.

It should also be pointed out that by a change in polarity of the charge of the particles 9 or of the source of voltage l4, positive images can be produced from negative masters and negative images from positive masters.

When the polarity of the electrode 1 is the same as the sign of the particle charge, a reversal process takes (ii I place, i.e. a negative image is produced from a positive master and a positive image from a negative master.

If the polarity of the electrode 1 is opposite the sign of the particle charge, no reversal process takes place. i.e. a positive master gives a positive image and a negative master gives a negative image.

Information will now be given as to the nature. composition and production of substances which have proven advantageous for the production of images by the present process.

1st Example l. Example of a photoconductor for the production of the photoconductive layer 4 A a'thick layer of a mixture of polyvinyl carbazole and 3 percent by weight tetranitrofluorenone as sensitizer.

lb. Example of a toner 48' (first color: yellow) 0.3 g Litholfast yellow 4R/l780 (BASFy as well as 0.04 g of an 8% solution of cobalt decanate in mineral spirits are made up with Isopar G (ESSO) to l()() g and dispersed for 10 min. in a high-frequency stirrer. In this way there is produced a yellow toner with positively charged particles.

lc. Example of a toner 48" (second color; magenta) 10 parts by weight Fanal pink 322/2994 (BASF) are ground together with 90 parts by weight of lsopar G in a bead ball mill for min. 4 g of this concentrate as well as 0.04 g of an 8'70 solution of cobalt decanatc in mineral spirits are made up to l()() g with lsopar G and dispersed in a high-frequency agitator for l() min. In this way there is produced a magenta toner with posi tively charged particles.

ld. Example of a toner 48" (third color: cyan) 5 parts by weight Savinyl blue GLS (Sandoz) and 5 parts by weight Irgazin blue 3 GT (CIBA-GEIGY) are ground together with 90 parts by weight of lsopar G in a head ball mill for 30 min. 6 g of this concentrate and 0.04 g of an 8% cobalt decanate solution in mineral spirits are made up of to 100 g with lsopar G and dispersed for H) min. in a high-frequency agitator. In this way there is produced a cyan toner with positively charged particles.

le. Example of a wetting liquid 39 As wetting liquid there is preferably used the same liquid as employed to produce the toner, for instance. a hydrocarbon of high insulating power such as lsopar G (ESSO).

le. Example for the production of the picture with substances indicated under la to le.

After the wetting of the support 6 with the wetting liquid 39, the two electrodes 1 and 5 are aligned face to face and the intervening space 7 is filled with the yellow toner 48'. The master I] is projected for 6 see. through a violet filter onto the photoconductor layer 4 and during the same time a voltage of-2.S kv is applied to the NESA layer 3 of electrode 1. After the superficial drying of the first partial image and the cleaning of the photoconductor 4, the magenta toner 48" is introduced. the master II is projected through a green filter for 6 seconds and at the same time a voltage of 3 kv is applied to the NESA layer 3 of electrode 1. After again drying and cleaning. the cyan toner 48" is introduced. the master 11 is projected through a red filter for 6 sec. and at the same time a voltage of l .5 kv is applied to the NESA layer 3 of electrode 1. The dis tance d between the surface of the paper and the surface of the photoconductor is I60 1.1.. After the drying and treatment with the fixing agent. a positive full-color image corresponding to the master II is produced on the support 6.

2nd Example 2a. Example of a photoconductor 4 B-phthalocyanin is mixed with a styrene-acrylic resin as binder and applied by coating in the form ofa layer of a thickness of 5 a. The pigment/binder ratio is 3: l 0.

2b. Example of a toner 48' (color black) 5 ml of Hunt toner concentrate (HUNT CHEMICAL CORPORATION) are diluted to [00 ml with Isopar G and mixed well by agitation. In this way there is produced a black toner with positively charged particles.

2c. Example of the production of the image A black-white positive is projected as master 1] for 6 see. without color filter onto the photoconductor layer 4 and at the same time a volate of 3 kv is applied to the NESA layer 3 of electrode 1. When using the toner 48' of 2b there is produced on the support 6 a positive black-white image with good half tones which corresponds to the master 1]. The distance d is 200 a.

3rd Example 3a. Example of a photoconductor 4 Same as under la.

3b. Example of a toner 48' (first color: yellow) 2 g of the printing color paste Nagra 6" (SICPA) as well as 0.04 g of 8% cobalt deeanate solution is spirits of turpentine are made up to g with lsopar G and dispersed for 10 min. in a high-frequency agitator. A yellow toner having positively charged particles is produced.

3c. Example of a toner 48" (second color: magenta) 2 g of the printing color paste Nagra red 28" (SICPA) as well as 0.04 g of 8% cobalt decanate solution in spirits of turpentine are made up to I00 g with Isopar G and dispersed for 10 min. in a high-frequency agitator. A magenta toner having positively charged particles is produced.

3d. Example of a toner 48" (third color: cyan) 2 of the printing color paste Nagra blue 9 (SICPA) as well as 0.04 g of 8% cobalt decanate solution in spirits of turpentine are made up to 100 g with Isopar G and disperesed for 10 min. in a high-frequency agitator. A cyan toner having positively charged particles is produced.

3e. Example of a wetting liquid 39 Same as under 1d.

3f. Example for the preparation of an image with the substances indicated under 3a to Be.

After the wetting of the image receiver with the wetting liquid 39, the two electrodes I and 5 are aligned face to face and the space 7 is filled with the yellow toner 48'. The master 1] a colored slide is projected for 6 see. through a violet filter onto the photoconductor layer 4 and a voltage of 2 kv is applied during the same time with the NESA layer 3 of electrode 1. After the drying of the surface of the first partial image and the cleaning of the photoconductor 4, the magenta toner 48" is introduced. the master 11 is projected through a green filter for 6 sec. and at the same time a voltage of-l .8 kv is applied to the NESA electrode. After again drying and cleaning. the cyan toner 48" is introduced, the master 1 l is projected for 6 sec. through a red filter. and at the same time a voltage of *-().9 kv is applied to the NESA layer 3 of electrode 1.

The distance d is 200 a. After the drying and treating with the fixing agent there is produced on the support 6 a positive full-color image which corresponds to the master 11.

We claim:

1. A method of producing images in accordance with a master, comprising the steps of:

a. providing a photoconductive layer spaced from an image support adjacent thereto, and electrodes respectively associated therewith,

b. bringing a dispersion of charged toner particles into the space between said layer and said support, while shunting the electrodes,

exposing the photoconductive layer to a light image of the master, thereby producing with respect to said layer, a conductivity image corresponding to the master,

d. applying an electric field across said space by means including the electrodes during at least part of the exposure, said field causing migration of the charged toner particles in a first direction of the charged toner particles with change of toner particle charge as controlled by said conductivity image, and

e. again shunting the electrodes, whereby a further migration of toner particles occurs in a second direction to form a visible image of the master on the image support.

2. Method according to claim 1 characterized by the fact that step c, the exposing of the photoconductive layer, is effected with a color separation of the master, and the toner brought into said space in step (b) is suitably adapted to the color separation, whereby a partial color image of the master is produced.

3. Method according to claim 2, characterized by the fact that by repetition of steps (b) through (e) with a plurality of successive discrete color separations and respective toners adapted thereto, successive partial color images are formed on the same image support, a full color image being produced by superimposing the successive partial color images.

4. method according to claim 3, characterized by the fact that for each color separation a toner of complementary color to same is used in the related step (b).

5. Method according to claim 3, characterized by the fact that first of all a yellow partial color image is pro duced and then a magenta partial color image and finally a cyan color partial image, these color partial images being superimposed on each other on the same support.

6. Method according to claim 3, characterized by the fact that, as a further step f, each partial image produced on the support is dried on its surface and preferably after removal of excessive toner as an intervening step (e), the same support then being used in a further cycle of steps (b) through (f) for the production of another partial image until a full color image is obtained.

7. Method according to claim 1, characterized by the fact that the polarity of said electrode associated with the photoconductive layer is opposite the polarity of said charged toner particles brought into said space, whereby, in the case of a positive master, a positive image is produced, and in the case ofa negative master, a negative image is produced.

8. Method according to claim 1, characterized by the fact that the polarity of said electrode associated with the photoconductive layer is the same as the polarity of the toner particles brought into said space, whereby, in the case of a positive master, a negatige image is produced, and in the case of a negative master, a positive image is produced.

9. Method according to claim 1, characterized by the fact that paper is used as support for the image.

10. Method according to claim 1, characterized by the fact that plastic is used for the support for the image.

11. Method according to claim 1, characterized by the fact that glass is used as support for the image.

12. Method according to claim 1, characterized by the fact that metal is used as support for the image.

13. Method according to claim 1, characterized by the fact that textile material is used as support for the image.

14. Method according to claim 1, characterized by the fact that the support for the image is wetted before the introduction of the toner into said space.

15. Method according to claim 14, characterized by the fact that said toner particles are dispersed in a discrete solvent and further including the step of wetting the support with a wetting liquid of the same type as the solvent of the toner before said image support comes into contact with the toner,

16. Method according to claim 1, characterized by the fact that, as a further step (f), the support bearing the image is coated in a thin layer, preferably after prior drying as an intervening step (c), with an agent increasing the transparency and durability of the image.

17. Method according to claim 16, wherein the said agent is taken from the group consisting of lacquers, resins, low-melting polymers, and waxes.

18. In an electrophotographic apparatus having imaging mcans (12) including a light path (17) for forming a light image of a master (11); a first electrode (1) and a second electrode (5) spaced therefrom; and a voltage source (14); the combination of a switch (13) having two positions, said electrodes being connected across said voltage source with the switch in one position and connected to each other with the switch in the other position,

a photoconductive layer (4) associated with said first electrode (I) and adapted to form a conductivity image corresponding to the master upon exposure of said layer (4) to said light image,

an image support (6) arranged on the second electrode (5), and

a fluid [8) having charged toner particles dispersed therein and being located in the space (7) between the two electrodes, said charged toner particles undergoing a an initial migration in a first direction in response to placing the switch in said one position during at least a part of said exposure of the photoconductive layer to the light image and undergoing a further migration in response to subsequent placing of the switch in a second direction in said other position, whereby a toned image is formed on said image support.

19. Apparatus according to claim 18, characterized by imaging means (12) having a shutter (16) settablc to given exposure times.

20. Apparatus according to claim l8, characterized by color filters (l8, I9. 20) selcctably insertable into said light path (17).

21. Apparatus according to claim 20, characterized by said color filters being color separation filters such as red L599, violet U438, green U525.

22. Apparatus according to claim 18, characterized by a gray filter (21) selectably insertable in said light path (17).

23. Apparatus according to claim 18, characterized by a mirror (22) which deflects said light path (17) towards the photoconductive layer (4).

24. Apparatus according to claim 18, characterized by said first electrode (1) being a transparent conductive layer (3) applied to a glass plate (2).

25. Apparatus according to claim 24, characterized by a layer (3) of tin oxide as electrode (1).

26. Apparatus according to claim 18, characterized by the photoconductive layer (4) being applied over a transparent electrode (1).

27. Apparatus according to claim 18, characterized by second electrode (5) being arranged equidistant from the first electrode (1 said first electrode l having the photoconductive layer (4) arranged thereon.

28. Apparatus according to claim 18, wherein the photoconductive layer (4) has a surface facing the support (6) and the support (6) has a surface facing the photoconductive layer (4), and characterized by a distance d of less than 2 mm between said facing surfaces.

29. Apparatus according to claim 18, characterized by an arrangement of the first and second electrodes (1, S) such that they are movable with respect to each other.

30. Apparatus according to claim 29, characterized by the fact that the movability is limited by an adjustable stop (31).

31. Apparatus according to claim 18, characterized by an arrangement of photoconductive layer (4) and second electrode (5) such that they are movable with respect to each other.

32. Apparatus according to claim 18, wherein a displaceable holding device (26) receives image support (6) and second electrode (5). and further including guide means (27) for the displacement thereof.

33. Apparatus according to claim 18, characterized by a loading device (32) for loading the second electrode (5) with a support (6).

34. Apparatus according to claim 18, characterized by a wetting device (37) for the support (6), said wetting device being movable relative thereto during production of an image.

35. Apparatus according to claim 18, characterized by a toner feed device (46) for the feeding of at least one toner fluid (8 or 48', 48", 48") into the space (7) between the two electrodes (1, 5).

36. Apparatus according to claim 18, characterized by a holding device (26) for the second electrode (5), said holding device being movably supported.

37. Apparatus according to claim 18, characterized by means (61, 62) which serve to clean the photoconductive layer (4) after said further migration of toner particles.

38. Apparatus according to claim 37, wherein said means to clean the photoconductive layer (4) comprises a wiper (61) and a roller (62).

39. Apparatus according to claim 56, characterized by a cleaning device (18) for removing excess toner on the image support (6).

40. Apparatus according to claim 39, wherein said cleaning device (56) includes a shiftable roller, and characterized by the fact that a cleaning roller (62) is moved in a path past the photoconductive layer (4) for cleaning thereof, and the cleaning device (56) is arranged in the path of said cleaning roller (62) such that upon passage of the cleaning roller (62) past the cleaning device (56) the shiftable roller (58) thereof is shifted to a position such that it is touched by the passing cleaning roller (62) and thereby stripped of adhering toner.

41. Apparatus according to claim 18, characterized by a drying device (57) for at least drying the surface of an image on support (6).

42. Apparatus according to claim 18, characterized by a control device for at least partially automatic control of the course of image production.

43. Apparatus according to claim 18, characterized by presence of a protective layer on photoconductive layer (4), said protective layer being of material more wear resistant than the photoconductive layer, said material being taken from the group consisting of Teflon. SiO and silicon nitride.

44. The herein-described electrophotograph obtained by a. providing a photoconductive layer spaced from an image support adjacent thereto, and electrodes respectively associated therewith,

b. bringing a dispersion of charged toner particles into the space between said layer and said support, while shunting the electrodes,

0. exposing the photoconductive layer to a light image of the master. thereby producing with respect to said layer, a conductivity image corresponding to the master,

(1. applying an electric field across said space by means including the electrodes during at least part of the exposure, said field causing migration of the charged toner particles in a first direction with change of toner particle charge as controlled by said conductivity image, and

e. again shunting the electrodes, whereby a further migration of toner particles occurs in a second direction to form a visible image of the master on the image support, and

f. fixing said visible image on said image support.

45. The electrophotograph of claim 44, wherein a full color image is obtained by repetitive cycling of steps b through c with the same image support. a discrete color separation being provided in step (c) of each cycle. and a respective toner adapted thereto being used in step b of the related cycle, thereby forming partial color images superimposed on the image support to yield the desired full color image. 

1. A METHOD OF PRODUCING IMAGES IN ACCORDANCE WITH A MASTER, COMPRISING THE STEPS OF: A. PROVIDING A PHOTOCONDUCTIVE LAYER SPACED FROM AN IMAGE SUPPORT ADJACNT THERETO, AND ELECTRODES RESPECTIVELY ASSOCIATED THEREWITH, B. BRINGING A DISPERSION OF CHARGED TONER PARTICLES INTO THE SPACE BETWEEN SAID LAYER AND SAID SUPPORT, WHILE SHUNTING THE ELECTRODES, C. EXPOSING THE PHOTOCONDUCTIVE LAYER TO A LIGHT IMAGE OF THE MASTER, THEREBY PRODUCING WITH RESPECT TO SAID LAYER, A CONDUCTIVE IMAGE CORRESPONDING TO THE MASTER, D. APPLYING AN ELECTRIC FIELD ACROSS SAID SPACE BY MEANS INCLUDING THE ELECTRODES DURING AT LEAST PART OF THE EXPOSURE, SAID FIELD CAUSING MIGRATION OF THE CHARGED TONER PARTICLES IN A FIRST DIRECTION OF THE CHARGED TONER PARTICLES WITH CHANGE OF TONER PARTICLE CHARGE AS CONTROLLED BY SAID CONDUCTIVITY IMAGE, AND
 2. Method according to claim 1 characterized by the fact that step c, the exposing of the photoconductive layer, is effected with a color separation of the master, and the toner brought into said space in step (b) is suitably adapted to the color separation, whereby a partial color image of the master is produced.
 3. Method according to claim 2, characterized by the fact that by repetition of steps (b) through (e) with a plurality of successive discrete color separations and respective toners adapted thereto, successive partial color images are formed on the same image support, a full color image being produced by superimposing the successive partial color images.
 4. method according to claim 3, characterized by the fact that for each color separation a toner of complementary color to same is used in the related step (b).
 5. Method according to claim 3, characterized by the fact that first of all a yellow partial color image is produced and then a magenta partial color image and finally a cyan color partial image, these color partial images being superimposed on each other on the same support.
 6. Method according to claim 3, characterized by the fact that, as a further step f, each partial image produced on the support is dried on its surface and preferably after removal of excessive toner as an intervening step (e), the same support then being used in a further cycle of steps (b) through (f) for the production of another partial image until a full color image is obtained.
 7. Method according to claim 1, characterized by the fact that the polarity of said electrode associated with the photoconductive layer is opposite the polarity of said charged toner particles brought into said space, whereby, in the case of a positive master, a positive image is produced, and in the case of a negative master, a negative image is produced.
 8. Method according to claim 1, characterized by the fact that the polarity of said electrode associated with the photoconductive layer is the same as the polarity of the toner particles brought into said space, whereby, in the case of a positive master, a negatige image is produced, and in the case of a negative master, a positive image is produced.
 9. Method according to claim 1, characterized by the fact that paper is used as support for the image.
 10. Method according to claim 1, characterized by the fact that plastic is used for the support for the image.
 11. Method according to claim 1, characterized by the fact that glass is used as support for the image.
 12. Method according to claim 1, characterized by the fact that metal is used as support for the image.
 13. Method according to claim 1, characterized by the fact that textile material is used as support for the image.
 14. Method according to claim 1, characterized by the fact that the support for the image is wetted before the introduction of the toner into said space.
 15. Method according to claim 14, characterized by the fact that said toner particles are dispersed in a discrete solvent and further including the step of wetting the support with a wetting liquid of the same type as the solvent of the toner before said image support comes into contact with the toner.
 16. Method according to claim 1, characterized by the fact that, as a further step (f), the support bearing the image is coated in a thin layer, preferably after prior drying as an intervening step (e), with an agent increasing the transparency and durability of the image.
 17. Method according to claim 16, wherein the said agent is taken from the group consisting of lacquers, resins, low-melting polymers, and waxes.
 18. In an electrophotographic apparatus having imaging means (12) including a light path (17) for forming a light image of a master (11); a first electrode (1) and a second electrode (5) spaced therefrom; and a voltage source (14); the combination of a switch (13) having two positions, said electrodes being connected across said voltage source with the switch in one position and connected to each other with the switch in the other position, a photoconductive layer (4) associated with said first electrode (1) and adapted to form a conductivity image corresponding to the master upon exposure of said layer (4) to said light image, an image support (6) arranged on the second electrode (5), and a fluid (8) having charged toner particles dispersed therein and being located in the space (7) between the two electrodes, said charged toner particles undergoing a an initial migration in a first direction in response to placing the switch in said one position during at least a part of said exposure of the photoconductive layer to the light image and undergoing a further migration in response to subsequent placing of the switch in a second direction in said other position, whereby a toned image is formed on said image support.
 19. Apparatus according to claim 18, characterized by imaging means (12) having a shutter (16) settable to given exposure times.
 20. Apparatus according to claim 18, characterized by color filters (18, 19, 20) selectably insertable into said light path (17).
 21. Apparatus according to claim 20, characterized by said color filters being color separation filters such as red L599, violet U438, green U525.
 22. Apparatus according to claim 18, characterized by a gray filter (21) selectably insertable in said light path (17).
 23. Apparatus according to claim 18, characterized by a mirror (22) which deflects said light path (17) towards the photoconductive layer (4).
 24. Apparatus according to claim 18, characterized by said first electrode (1) being a transparent conductive layer (3) applied to a glass plate (2).
 25. Apparatus according to claim 24, characterized by a layer (3) of tin oxide as electrode (1).
 26. Apparatus according to claim 18, characterized by the photoconductive layer (4) being applied over a transparent electrode (1).
 27. Apparatus according to claim 18, characterized by second electrode (5) being arranged equidistant from the first electrode (1), said first electrode (1) having the photoconductive layer (4) arranged thereon.
 28. Apparatus according to claim 18, wherein the photoconductive layer (4) has a surface facing the support (6) and the support (6) has a surface facing the photoconductive layer (4), and characterized by a distance d of less than 2 mm between said facing surfaces.
 29. Apparatus according to claim 18, characterized by an arrangement of the first and second electrodes (1, 5) such that they are movable with respect to each other.
 30. Apparatus according to claim 29, characterized by the fact that the movability is limited by an adjustable stop (31).
 31. Apparatus according to claim 18, characterized by an arrangement of photoconductive layer (4) and second electrode (5) such that they are movable with respect to each other.
 32. Apparatus according to claim 18, wherein a displaceable holding device (26) receives image support (6) and second electrode (5), and further including guide means (27) for the displacement thereof.
 33. Apparatus according to claim 18, characterized by a loading device (32) for loading the second electrode (5) with a support (6).
 34. Apparatus according to claim 18, characterized by a wetting device (37) for the support (6), said wetting device being movable relative thereto during production of an image.
 35. Apparatus according to claim 18, characterized by a toner feed device (46) for the feeding of at least one toner fluid (8 or 48'', 4848'''''') into the space (7) between the two electrodes (1, 5).
 36. Apparatus according to claim 18, characterized by a holding device (26) for the second electrode (5), said holding device being movably supported.
 37. Apparatus according to claim 18, characterized by means (61, 62) which serve to clean the photoconductive layer (4) after said further migration of toner particles.
 38. Apparatus according to claim 37, wherEin said means to clean the photoconductive layer (4) comprises a wiper (61) and a roller (62).
 39. Apparatus according to claim 56, characterized by a cleaning device (18) for removing excess toner on the image support (6).
 40. Apparatus according to claim 39, wherein said cleaning device (56) includes a shiftable roller, and characterized by the fact that a cleaning roller (62) is moved in a path past the photoconductive layer (4) for cleaning thereof, and the cleaning device (56) is arranged in the path of said cleaning roller (62) such that upon passage of the cleaning roller (62) past the cleaning device (56) the shiftable roller (58) thereof is shifted to a position such that it is touched by the passing cleaning roller (62) and thereby stripped of adhering toner.
 41. Apparatus according to claim 18, characterized by a drying device (57) for at least drying the surface of an image on support (6).
 42. Apparatus according to claim 18, characterized by a control device for at least partially automatic control of the course of image production.
 43. Apparatus according to claim 18, characterized by presence of a protective layer on photoconductive layer (4), said protective layer being of material more wear resistant than the photoconductive layer, said material being taken from the group consisting of Teflon, SiO2 and silicon nitride.
 44. The herein-described electrophotograph obtained by a. providing a photoconductive layer spaced from an image support adjacent thereto, and electrodes respectively associated therewith, b. bringing a dispersion of charged toner particles into the space between said layer and said support, while shunting the electrodes, c. exposing the photoconductive layer to a light image of the master, thereby producing with respect to said layer, a conductivity image corresponding to the master, d. applying an electric field across said space by means including the electrodes during at least part of the exposure, said field causing migration of the charged toner particles in a first direction with change of toner particle charge as controlled by said conductivity image, and e. again shunting the electrodes, whereby a further migration of toner particles occurs in a second direction to form a visible image of the master on the image support, and f. fixing said visible image on said image support.
 45. The electrophotograph of claim 44, wherein a full color image is obtained by repetitive cycling of steps b through e with the same image support, a discrete color separation being provided in step (c) of each cycle, and a respective toner adapted thereto being used in step b of the related cycle, thereby forming partial color images superimposed on the image support to yield the desired full color image. 